The present invention relates to the radiography art. It finds particular application in conjunction with computerized tomographic (CT) scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention may also find application in conjunction with other radiation treatment apparatus and imaging apparatus.
Heretofore, tomographic scanners have commonly included a floor-mounted frame assembly which remains stationary during a scan. An x-ray tube is mounted to a rotatable frame assembly which rotates around a patient receiving examination region during the scan. Radiation from the x-ray tube traverses the patient receiving region and impinges upon an array of radiation detectors. From the radiation data sampled by the detectors and the position of the x-ray tube during each sampling, a tomographic image of one or more slices through the patient is reconstructed.
An x-ray tube generates x-rays by directing a high energy electron beam against a tungsten target. One of the persistent problems in CT scanners and other radiographic apparatus is dissipating the waste heat created while generating x-rays. In higher powered x-ray tubes, the anode turns so that the high energy electron beam only dwells a fraction of a second at a time on any point on the anode. The x-ray tube is jacketed with a lead lined housing. A cooling oil is circulated between the glass envelope of the x-ray tube and the housing to remove additional heat.
In some scanners, the x-ray tube rotates in one direction during a scan and returns in the other direction for the next scan. Such scanners are normally limited to about 360.degree. of rotation. The single rotation enables the hot cooling oil to be conveyed from the rotating frame by flexible hoses to a non-rotating heat exchanger. Accommodating the cooling oil-carrying hoses is a space consumptive handling problem. The heat exchanger is commonly a radiator disposed within the CT scanner housing that is cooled by fans which blow room air through the heat exchanger and back into the room. This places an extra load on the room air conditioning system.
In other CT scanners, the cooling oil is circulated to a radiator or other air-oil heat exchanger that is mounted on the rotating frame portion. This alleviates the hose handling problems and enables the x-ray tube to rotate a plurality of times, e.g. a continuous rotate scanner. However, accommodating the size and weight of the heat exchanger in the tight space constraints of the rotating frame is difficult. As the x-ray tube and rotating frame portion rotate, air passes through the heat exchanger cooling the oil. The heated air that is discharged into the room that contains the CT scanner again places a greater load on the room air conditioning system.
One of the limiting factors on the speed of a CT scan is the amount of x-rays produced by the x-ray tube. The tube must irradiate each detector for a sufficient duration that each detector receives the minimum total flux needed to reconstruct a good contrast image. Lower power tubes require the tube to dwell or focus longer on each detector. Larger, more powerful x-ray tubes supply the minimum flux more quickly, allowing the speed of rotation to be increased, hence the scan time decreased. However, as the x-ray tubes become more powerful, more heat is generated. More heat is also generated in continuous rotate scanners in which the tube remains "on" during several consecutive rotations for multislice imaging.
Larger x-ray tubes, such as seven inch anode x-ray tubes, generate so much heat that the prior art heat dissipation techniques are taxed. The limited air volume in the interior of a CT scanner limits the effectiveness of the rotating oil-air heat exchanger. Space constraints prevent larger heat exchangers from being accommodated on the rotating frame. Moreover, the added heat taxes the cooling capacity of room air conditioning systems to the point that room cooling capacity must be increased.
The present invention provides a new and improved cooling system which overcomes the above referenced problems and others.